1. Field of the Invention
The present invention relates to a process for producing reclaimed water and a system for producing reclaimed water whereby to obtain reclaimed water by using sewage effluent or various other types of treated wastewater as raw water.
2. Description of Related Art
To enhance efficient use of water resources, techniques for obtaining reclaimed water by subjecting various types of wastewater to membrane filtration have been developed. For example, Patent Documents 1 and 2 disclose methods for subjecting the washing wastewater in a filter basin of a water purifying plant to membrane filtration, thereby recycling the wastewater as washing water. In the case of the inventions described in these Patent Documents 1 and 2, the properties of wastewater are relatively good, but if wastewater contains a large quantity of fine solids, such as organic matters, as in the case of discharged sewage, then filtering the wastewater through a membrane causes the surface of the membrane to be fouled soon with fine solids, such as organic matters, in the wastewater, possibly leading to an operation failure.
As the techniques for preventing the membrane surface from fouling, there are methods whereby organic matters, which cause a membrane surface fouling, are decomposed by adding ozone to raw water, as disclosed in Patent Documents 3 and 4. However, a large quantity of ozone is required to fully decompose organic matters. An ozone generator, however, poses problems in that it is costly, consumes much electric power to generate ozone, and requires maintenance cost, resulting in high running cost.
Further, if a large quantity of ozone is added, then the decomposition of organic matters proceeds and the decomposed organic matters pass through a membrane, so that the concentration of the residual organic matters in the membrane-filtered water increases, causing a problem that the reclaimed water ends up with a deteriorated water quality. For these reasons, ozone has hardly been used to obtain reclaimed water by filtering wastewater through a membrane. The main objects of the inventions disclosed in both Patent Documents 3 and 4 are water purifying treatment process.
Patent document 5 describes a process for producing reclaimed water by subjecting secondary-treated sewage effluent to pre-ozone treatment process, biofiltration process, ozone treatment process, and membrane filtration process. This method, however, is not considered to be easily applicable to actual use because the method also requires a large quantity of ozone, and accidental inflow of the ozone from the pre-ozone treatment process to the biofiltration process caused by, for example, changes in the properties of raw water, would deteriorate the activity of the biofilm, possibly leading to a deteriorated quality of the treated water.